Re: Index Skip Scan - Mailing list pgsql-hackers

> On Wed, Sep 25, 2019 at 2:33 AM Dmitry Dolgov <9erthalion6@gmail.com> wrote:
> v27-0001-Index-skip-scan.patch
>
> Some random thoughts on this:

Thanks a lot for the commentaries!

> * Is _bt_scankey_within_page() really doing the right thing within empty pages?
>
> It looks like you're accidentally using the high key when the leaf
> page is empty with forward scans (assuming that the leaf page isn't
> rightmost). You'll need to think about empty pages for both forward
> and backward direction scans there.

Yes, you're right, that's an issue I need to fix.

> Actually, using the high key in some cases may be desirable, once the
> details are worked out -- the high key is actually very helpful with
> low cardinality indexes. If you populate an index with retail
> insertions (i.e. don't just do a CREATE INDEX after the table is
> populated), and use low cardinality data in the indexed columns then
> you'll see this effect.

Can you please elaborate a bit more? I see that using high key will help
a forward index scans to access the minimum number of leaf pages, but
I'm not following how is it connected to the _bt_scankey_within_page? Or
is this commentary related in general to the whole implementation?

> * The extra scankeys that you are using in most of the new nbtsearch.c
> code is an insertion scankey -- not a search style scankey. I think
> that you should try to be a bit clearer on that distinction in
> comments. It is already confusing now, but at least there is only zero
> or one insertion scankeys per scan (for the initial positioning).
>
> * There are two _bt_skip() prototypes in nbtree.h (actually, there is
> a btskip() and a _bt_skip()). I understand that the former is a public
> wrapper of the latter, but I find the naming a little bit confusing.
> Maybe rename _bt_skip() to something that is a little bit more
> suggestive of its purpose.
>
> * Suggest running pgindent on the patch.

Sure, I'll incorporate mentioned improvements into the next patch
version (hopefully soon).

> And now some more:
>
> * I'm confused about this code in _bt_skip():
>
Yeah, it shouldn't be there, but rather before _bt_next, that expects
unlocked buffer. Will fix.

> * It also seems a bit odd that you assume that the scan is
> "scan->xs_want_itup", but then check that condition many times. Why
> bother?
>
> * Similarly, why bother using _bt_drop_lock_and_maybe_pin() at all,
> rather than just unlocking the buffer directly? We'll only drop the
> pin for a scan that is "!scan->xs_want_itup", which is never the case
> within _bt_skip().
>
> I think that the macros and stuff that manage pins and buffer locks in
> nbtsearch.c is kind of a disaster anyway [1]. Maybe there is some
> value in trying to be consistent with existing nbtsearch.c code in
> ways that aren't strictly necessary.

Yep, I've seen this thread, but tried to be consistent with the
surrounding core style. Probably it indeed doesn't make sense.

> * Not sure why you need this code after throwing an error:
>
> >                 else
> >                 {
> >                     elog(ERROR, "Could not read closest index tuples: %d", offnum);
> >                     pfree(so->skipScanKey);
> >                     so->skipScanKey = NULL;
> >                     return false;
> >                 }

Unfortunately this is just a leftover from a previous version. Sorry for
that, will get rid of it.